1. Field of the Invention
The present invention relates to a sensing device for detecting the change in magnetic field caused by the motion of a moving member of magnetic material, and more particularly, to a sensing device which is particularly suitable for detecting the information about the rotation of for example an internal combustion engine.
2. Description of the Related Art
Magnetoresistance devices generally refer to those devices which change in resistance in response to the direction of a magnetic field applied to a thin ferromagnetic film with respect to the direction of a current flowing through the thin ferromagnetic film.
Magnetoresistance devices have minimum resistance when a magnetic field is applied in a direction at a right angle to the direction of current. On the other hand, when the angle between the direction of the current and the direction of the applied magnetic field is 0, that is when a magnetic field is applied in a direction same as or opposite to the direction of current, the resistance has a maximum value. The change in the resistance is generally called the magnetoresistance effect, and the magnitude of the change in the resistance is referred to as the magnetoresistance variation ratio. A typical value of magnetoresistance variation ratio is 2 to 3% for Ni-Fe and 5 to 6% for Ni-Co.
FIG. 20 is a schematic diagram illustrating the construction of a conventional sensing device, wherein its side view and perspective view are shown in FIG. 20a and FIG. 20b, respectively.
The sensing device shown in FIG. 20 includes: a rotating shaft 1; a rotary member of magnetic material 2 having at least one protruding or recessed portion wherein the rotary member of magnetic material 2 is adapted to rotate in synchronization with the rotation of the rotating shaft 1; a magnetoresistance device 3 disposed at a location a predetermined distance apart from the rotary member of magnetic material 2; and a magnet 4 for applying a magnetic field to the magnetoresistance device 3. In the above construction, the magnetoresistance device 3 includes a magnetoresistance pattern 3a and a thin film surface (magnetic field sensing plane) 3b.
If the rotary member of magnetic material 2 rotates, the magnetic field applied to the magnetic field sensing plane 3b of the magnetoresistance device 3 changes in response to the rotation of the rotary member of magnetic material 2, and, as a result, the resistance of the magnetoresistance pattern 3a changes correspondingly.
FIG. 21 is a block diagram illustrating the construction of the sensing device using the magnetoresistance devices described above.
The sensing device includes: a Wheatstone bridge circuit 11 including magnetoresistance devices disposed at a predetermined distance apart from the rotary member of magnetic material 2 so that a magnetic field is applied from a magnet 4 to the magnetoresistance devices; a differential amplifier 12 for amplifying the output signal of the Wheatstone bridge circuit 11; a comparator 13 for comparing the output of the differential amplifier 12 with a reference values V.sub.T1 , V.sub.T2 and outputting a "0" signal or a "1" signal depending on the comparison result; a waveform shaping circuit 14 for shaping the waveform of the output of the comparator 13 and supplying a "0" or "1" signal having a sharp rising and falling edges to the output terminal 15.
The operation will be described below with reference to FIG. 22.
If the rotary member of magnetic material 2 rotates, the magnetic field applied to each of the magnetoresistance devices changes in response to the passage of the protruding and recessed portions of the rotary member of magnetic material 2 as shown in FIG. 22a. As a result, the above change in the magnetic field is detected by the magnetoresistance devices, mid-point voltages) of the Wheatstone bridge circuit 11 also change in a similar fashion.
The difference between the mid-point voltages is amplified by the differential amplifier 12. Thus, as shown in FIG. 22b, the differential amplifier 12 outputs a signal V.sub.D0 corresponding to the passage of the protruding and recessed portions of the rotary member of magnetic material 2 shown in FIG. 22a.
The comparator 13 compares the output signal V.sub.D0 of the differential amplifier 12 with the reference values V.sub.T1 , V.sub.T2 and outputs a "0" or "1" signal in response to the comparison result. The output signal of the comparator 13 is shaped by the waveform shaping circuit 14 so that a "0" or "1" output signal having sharp rising and falling edges is provided to the output terminal 15 as shown in FIG. 22c.
However, the conventional sensing device having the above construction has the problem that the change in characteristics with temperature can result in degradation in accuracy in the detected signal and deviation can occur in the output signal relative to the location of the protruding and recessed portions of the rotating member of magnetic material.
That is, MR devices broadly employed in conventional sensing devices have a large temperature coefficient compared with usual resistors, and thus exhibit a great change in resistance with temperature. Furthermore, the temperature coefficient itself changes with resistance, that is, the temperature coefficient when the MR device has a maximum resistance is different from that obtained when the MR device has a minimum resistance. More specifically, while usual resistors typically have a rather small temperature coefficient in the range from 300 to 400 ppm/.degree.C., the temperature coefficient of the MR devices is as great as about 2500 ppm/.degree.C. for the maximum resistance condition and about 2700 ppm/.degree.C. for the minimum resistance condition.
Therefore, if the Wheatstone bridge circuit 11 is constructed with MR devices as shown in FIG. 21, the mid-point voltage of the Wheatstone circuit 11 changes in response not only to the change in resistance of the MR devices caused by the change in magnetic field but also to the temperature coefficients of the MR devices. Furthermore, the difference in the temperature coefficient among the MR devices constituting the bridge circuit also produces a change in the mid-point voltage.
For example, if the characteristic shown in FIG. 22 at an ambient temperature of T.sub.0 (for example room temperature) changes due to an increase in the ambient temperature from T.sub.0 to T.sub.1, a great difference can occur between the voltages of mid-points of the Wheatstone bridge circuit 11. This difference results, as shown in FIG. 23b, in a great deviation of the output V.sub.D1 of the differential amplifier 12 which should correspond to the protruding and recessed portions of the rotating member 2 of magnetic material shown in FIG. 23a.
That is, the output V.sub.D1 of the differential amplifier 12 at temperature T.sub.1 becomes greater than that at temperature T.sub.0, or V.sub.D1 &gt;VD.sub.0 and the output V.sub.D1 is no longer within the allowable range relative to the reference voltages V.sub.T1, V.sub.T2. As a result, as shown in FIG. 23c, the output of the waveform shaping circuit 14 also deviates from the correct waveform represented by the broken lines in the figure, and thus the output signal no longer precisely corresponds to the protruding and recessed portions of the rotating member 2 of magnetic material.
It is a general object of the present invention to solve be above problems. More specifically, it is an object of the present invention to provide a sensing device capable of outputting a correct signal precisely corresponding to a particular position (angle) such as a protruding or recessed portion of a rotating member made of a magnetic material over the entire operating temperature range regardless of the temperature coefficient of the magnetic field sensing element.